1. Field of the Invention
This invention relates broadly in the field of electronic musical tone generators and in particular is concerned with the provision for a digital frequency follower for a musical instrument.
2. Description of the Prior Art
A problem commonly encountered in signal processing systems is to provide a means for determining the fundamental frequency of a complex input periodic signal. Sometimes the object is to simply determine the fundamental frequency while other times the determined fundamental frequency is used as an input to other systems. An example of the second system object is found in the variety of musical devices which are called by the generic name of "frequency followers."
The tone changer is an example of a frequency follower system. In a tone changer, the acoustic signal from a musical instrument such as a flute or saxaphone is converted into an electrical signal by means of a microphone that is usually inserted into a hole drilled in the musical instrument. Analog circuitry is used to force an oscillator to operate at the current fundamental frequency played on the musical instrument. The signal produced by this oscillator is then used as an input frequency control signal to an electronic tone synthesizer. The tone synthesizer usually operates at the fundamental frequency of the oscillator or at suboctaves which are readily obtained by means of conventional frequency dividers operating on the oscillator's output signal. The net effect is that the musician plays his acoustic instrument in the usual fashion while the frequency follower and tone synthesizer combination system provide an accompaniment which has a different and selectively adjustable tone color and may be selectively at the unison pitch or at some suboctave.
The analog frequency determining element used in tone changers is generally selected as some variation of a phase locked oscillator. Such devices work best when the input signal approximates a simple periodic waveshape such as a sinusoid shape. For this reason, tone changers using frequency followers have been most successful when used in conjunction with musical instruments having tone colors containing relatively few harmonics. For acoustic musical instruments having tones with an extended harmonic structure, a low pass filter is often employed prior to the phase locked oscillator so that the higher harmonics are attenuated to produce a simpler signal.
The use of a low pass filter to reduce tonal complexity places a musical limitation on the tone changer. With a filter it is necessary for the musician to preselect the octave ranges that will be played.
A common problem shared by frequency followers is the time required for the frequency determination system to change frequency in response to a change in the frequency of the input signal.
It is a feature of this invention that a digital frequency follower is used in a novel manner to provide the functions previously obtained using analog circuitry without some of the limitations encountered with state of the art frequency follower and tone changer combination systems.